A conventional traction elevator system includes a car, a counterweight, two or more ropes interconnecting the car and counterweight, a traction sheave to move the ropes, and a machine to rotate the traction sheave. The ropes are formed from laid or twisted steel wire and the sheave is formed from cast iron.
Although conventional steel ropes and cast iron sheaves have proven very reliable and cost effective, there are limitations on their use. One such limitation is the traction forces between the ropes and the sheave. Typical techniques to increase the traction forces between the ropes and sheave result in reducing the durability of the ropes, increasing wear or the increasing rope pressure.
Another limitation on the use of steel ropes is the flexibility and fatigue characteristics of steel wire ropes. The minimum diameter of a steel rope is dictated mostly by fatigue requirements and results in a relatively thick rope. The relatively thick cross section of a steel rope reduces its inherent flexibility necessitating a sheave having a relatively large diameter. The larger the sheave diameter, the greater torque required from the machine to drive the elevator system thereby increasing the size and cost of the elevator system.
Another drawback of conventional round ropes is that smaller sheave diameters increase rope pressure shortening the life of the rope. Rope pressure is generated as the rope travels over the sheave and is directly proportional to the tension in the rope and inversely proportional to the sheave diameter D and the rope diameter. In addition, the shape of the sheave grooves, including such traction enhancing techniques as undercutting the sheave grooves, further increases the maximum rope pressure to which the rope is subjected.
In a typical rope driven elevator installation rope wedge clamps are used for termination purposes. Wedge clamps operate by securing the elevator rope between opposed angled walls of the wedge clamps and a tear drop shaped wedge around which the cable is wound. The wedge acts to cam the rope against the walls of the wedge clamp during tensioning of the ropes. A benefit of this design is that the wedge may have a relatively sharp angle producing a large clamping force. Because the steel ropes have a high compressive strength the large clamping force has no deleterious effects on the rope such as crush or creep.
In attempts to overcome the deficiencies and drawbacks of conventional round steel ropes for use in elevator systems coated tension members, including a relatively flat tension member, has been developed. The flat tension member includes a plurality of individual load carrying cords encased within a common layer of coating. An exemplary tension member of the type contemplated in this application is discussed in further detail in U.S. Ser. No. 09/031,108 filed Feb. 26, 1998 Entitled Tension Member For An Elevator and Continuation-In-Part Application Entitled Tension Member For An Elevator filed Dec. 22, 1998, both of which are entirely incorporated herein by reference.
The coating layer surrounds and/or separates the individual cords and defines an engagement surface for engaging a traction sheave. As a result of the configuration of the tension member, the rope pressure may be distributed more uniformly throughout the tension member, traction is increased and smaller sheave diameters are possible.
A method of terminating and securing flat tension members involves looping the members over a bar and clamping the end with a pair of plates. The plates are secured by a plurality of fasteners that pass through holes provided in the plates. Another method of terminating flat tension members includes a wedged end fastener wherein a wedge of material is positioned at the end of the tension member and clamped by a pair of plates. In such a configuration one of the plates comprises a wedge shaped cross section cooperating with the wedge of material and the second plate comprises a cross section of uniform thickness. The plates are similarly secured by a plurality of fasteners that pass through holes provided in the plates. A drawback to these types of termination methods is that the tension carrying capability of the termination relies solely on the clamping forces provided by the fasteners. In addition, the wedge type fastener limits the termination point of the member and hampers adjustability.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop more efficient and durable methods and apparatus to drive elevator systems.